fix: ensure version is defined when using $app/env with explicit env vars

.changeset/wide-otters-shout.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: ensure `version` is defined when importing from `$app/env` with explicit environment variables

packages/kit/src/runtime/app/env/index.js

@@ -1,2 +1,5 @@
 export { BROWSER as browser, DEV as dev } from 'esm-env';
 export { building, version } from './internal.js';
+
+// force the Vite client to load, so that defines are definitely defined
+import.meta.hot;

packages/kit/test/apps/options-2/src/hooks.client.js

@@ -0,0 +1,6 @@
+import { version } from '$app/env';
+
+// Regression guard for #15971: this import runs before the router is initialized.
+// Without the fix it throws `__SVELTEKIT_APP_VERSION__ is not defined` when
+// `experimental.explicitEnvironmentVariables` is enabled. `void` keeps the import.
+void version;

packages/kit/test/apps/options-2/test/test.js

@@ -184,3 +184,17 @@ test.describe("bundleStrategy: 'single'", () => {
 		await expect(page.locator('h1', { hasText: 'It works!' })).toBeVisible();
 	});
 });
+
+test.describe('$app/env', () => {
+	// regression test for https://github.com/sveltejs/kit/issues/15971:
+	// importing `$app/env` before the router is initialized (e.g. in
+	// hooks.client.js) must not throw `__SVELTEKIT_APP_VERSION__ is not defined`
+	test('version is defined when imported during client init', async ({ page }) => {
+		/** @type {string[]} */
+		const errors = [];
+		page.on('pageerror', (error) => errors.push(error.message));
+		await page.goto('/basepath');
+		await page.waitForTimeout(500);
+		expect(errors.filter((message) => message.includes('__SVELTEKIT_APP_VERSION__'))).toEqual([]);
+	});
+});